C's latest obsession is prime numbers.

He carries on about them at length, so much so that I installed an app on my phone that is nothing but a long list of prime numbers. He scrolls up and down, scanning for the red dots that mark a prime number, bouncing with joy when he discovers a new one.

He tells us that "two" is his favorite prime number because it's the only even prime number. He informs us that prime numbers are only divisible by one and themselves (as if we weren’t the ones who explained that to him in the first place). He's just starting to understand what "divisible" even means: "Eight isn't prime because two and four go into it."

He quizzes us: "Is 35 prime?" We play the mark: "Why, yes! 35 is prime!" He squeals with delight: "NO! 35 is NOT prime! But 37 IS prime!" So proud to be schooling mom and dad.

Tonight I asked him which of his classmates is a prime number. This stumped him, so I explained that if they had a prime number of letters in their name, they were prime. We determined that he's the only prime number in his class, a fact that pleased him greatly.

I don't really know how he first heard about prime numbers, and I don't know what the allure is.

I do know he loves them, so I love them now, too.

...

My favorite prime number, in case you were wondering, is 67...and I don't really know why.